Optical microscopes that provide phase-measuring capability are useful for resolving small surface height variations on an inspection sample and for distinguishing sample materials based on their reflectance phase properties. Examples of phase-sensitive microscopes are Mirau interference microscopes, Nomarski differential phase contrast microscopes, and heterodyne interference microscopes. In each of these microscopes, an illumination beam is separated into two beams by some type of beam-splitting mechanism; at least one of the beams is focused onto and reflects off an inspection sample; and the two beams are then coherently recombined by the beam-splitting mechanism and projected onto an optical detector. The phase relationship between the recombined beams affects the detector-plane beam intensity; thus the detector signal is sensitive to small path length differences or sample-induced phase differences between the two beams. Some systems are also provided with a phase-modulating mechanism that applies a controlled phase shift to the beams, and the signal is sampled at multiple phase shifts to enhance measurement sensitivity.
The beam-splitting mechanism in a Mirau-type microscope is a partially-reflecting mirror between the microscope objective and the sample. In this system one of the separated beams is focused onto the sample and the other is focused onto a small reference mirror proximate to the objective. A limitation of the Mirau microscope is that the phase relationship between the recombined beams depends on the sample""s focus height and is hence very sensitive to vibration and focus drift.
The Nomarski microscope focuses both beams onto the sample so that focus shifts affect both beams equally and therefore have no effect on their relative phase. In this type of system the beam-splitting element is typically a Wollaston prism which induces a slight angular separation between two polarization components of a transmitted beam, and the polarization-separated beams are focused by a microscope objective onto two proximate points on the sample. A Nomarski microscope is sensitive to differences in surface height or reflectance phase between the two focus points.
A heterodyne interference microscope is similar to a Nomarski microscope, except that the beam-splitting mechanism is typically an acousto-optic diffraction grating rather than a polarization-separation device. The grating functions to separate an incident beam into two diffracted beams. It also functions to introduce a precisely controlled frequency difference between the separated beams (which is equivalent to a time-variable induced phase shift). This greatly enhances the system""s phase-measuring capability.
Surface height variations can also be accurately measured by means of confocal microscopy, in which a pinhole-filtered beam is focused onto a sample and the reflected beam is filtered through the same pinhole and projected onto a detector. Typically, the focused beam is raster-scanned across the sample to synthesize a two-dimensional image, and the sample""s focus height may also be scanned to form a three-dimensional surface profile image. Confocal microscopes do not commonly provide phase-measuring capabilityxe2x80x94their depth discrimination is based on their narrow depth of focus. However, confocal microscopy can be used in conjunction with phase-measuring methods to provide enhanced depth discrimination. Several confocal systems of this type are described in xe2x80x9cConfocal Microscopyxe2x80x9d, ed. T. Wilson, Academic Press, 1990.
Confocal systems that use a single scanning spot tend to be limited in their image acquisition speed. This limitation can be overcome by using multiple parallel-scanning spots (e.g., by using a Nipkow disk scanner) or a high-speed scanner (such as an acousto-optic scanner). However, a limitation of all of these methods is that their field size is limited by the microscope objective. A confocal system can only achieve high depth discrimination by using an objective of high numerical aperture, and such objectives generally have comparatively small field sizes.
High-resolution, high-speed confocal imaging over large image fields can be achieved by using a scanning microlens array. A system of this type is described in xe2x80x9cThree-dimensional analysis by a microlens-array confocal arrangementxe2x80x9d, by Hans J. Tiziani and Hans-Martin Uhde, Applied Optics Vol. 33, No. 4 (1994), pp. 567-572; and a similar system is disclosed in U.S. pat. application Ser. No. 08/803,096. However, these systems do not provide phase-measuring capability.
A scanning confocal microlens array can be adapted to provide phase-measuring capability by equipping each microlens with a beam-splitting mechanism that separates the illumination on the microlens into two beams, at least one of which is focused onto and reflects off an inspection sample. The two beams are coherently recombined by the beam-splitting mechanism and are projected onto an element of a detector array, wherein each detector element senses radiation from a particular corresponding microlens. The sample is scanned laterally across the focal point array to build up a synthesized, two-dimensional image of the sample surface. The sample""s focus height may also be scanned to form a three-dimensional surface profile image. (Either the lateral or focus scanning, or both, may eqivalently be effected by moving the microlens array relative to the sample.)
The microlens array could also be equipped with a phase-modulating mechanism to provide enhanced phase-measuring capability, in a manner analogous to a confocal heterodyne microscope. However, the same objective can be more simply achieved by effectively using the scanning motion itself as the phase-modulating mechanism. In this mode of operation, each beam-splitting mechanism applies a fixed, built-in phase shift to the two separated beams, but the phase shift is not the same for all microlenses. Each object point on the sample is scanned by several microlenses that have different built-in phse shifts, and the detector signals acquired from the different scans are combine provide confocal image data over a range of phase shifts.